(1) Field of Invention
This invention is related to development of acoustic sensors configurations and methods for efficiently recording subsurface seismic data and more particularly to utilization of acoustic sensors for recording borehole seismic data.
(2) Background Art
Borehole seismic data can be utilized to refine surface seismic data prior to drilling production wellbores. Borehole seismic data can further be gathered on a continuing or recurrent basis to monitor subsurface formations and reservoirs during production of the well. The gathering of data on a continuing basis will assist in optimizing extraction of gas or oil deposits.
Borehole seismic surveys are conducted by placing the receivers in the borehole and operating a seismic source at the surface to generate an acoustic wave. Typically the receivers are placed in a shuttle and deployed downhole for the duration of the survey and then removed. The amount of information that can be obtained in borehole seismic surveys can be limited by the logistics of deploying the shuttles downhole.
It is known for acoustic sensors or receivers to be permanently deployed downhole to continuously monitor seismic data during production of a well. The sensors are typically deployed with a monitoring tool that extends downhole and is integrally attached to the borehole casing. The attachment means is typically a mechanical surface force clamping device and the sensors are typically housed in a side passageway or lateral extending section associated with the sensor housing or production tubing which is laterally displaced from the primary flow passageway through the production tubing. See, for example, U.S. Pat. No. 6,253,848 issued Jul. 3, 2001 to Reimers et al. The permanent deployment monitoring tooling such as that taught in Reimers et al, cannot typically be retrieved or removed without destroying the wellbore rendering the tool and sensors unusable for future borehole seismic operations.
Many monitoring tools for permanently deploying seismic sensor arrays downhole are single level monitoring tools. However, due to the complex subsurface formation and strata and the various levels of the multiple production zones and reservoirs, multilevel monitoring tools are also required to monitor various levels simultaneously. The monitoring tool that deploys the sensor arrays will typically include a plurality of sensor housings or shuttles where each shuttle contains at least one sensor. While a plurality of shuttles is desirable, an excessive number of shuttles can result in an overly complex tool that is very large and difficult to deploy. The total number of shuttles is typically eventually limited by the general power consumption requirements of the downhole sensor, telemetry and clamping system. In general, a tool based on the general tool architecture as outlined above can quickly become large and complex when trying to increase the number of shuttles, resulting in system that is both expensive and difficult to deploy. Due to system cost and high lost-in-hole risks, it can be impractical to deploy such a system permanently in a well. The number of shuttles is also limited due to power consumption requirements, costs and difficulty of deployment. Known borehole tools, including those utilizing fiber optic sensors, designed for permanently deploying sensor arrays typically include a surface force clamp attachment means for attaching the sensor arrays to the borehole casing. This type of attachment means results in a monitoring tool that is not retractable or reusable at a different site. A borehole sensing apparatus that is not easily removably deployed into a borehole and which cannot be retrieved and reused in other boreholes is a problem that exists.
Similarly in the area of borehole logging, the number of transmitters and receivers and the distance between transmitters and receivers has been increasing to improve the ability to detect formation characteristics in the undisturbed formation farther from the borehole. One method to get deeper penetration is to increase the distance between source and receivers, such that the receivers are detecting signals that are returned from further distances in the borehole. A problem with increasing the distance between sources and receivers is that increasing tool size can result in increasing difficulties in deployment, longer periods of time required for logging, longer down-time for the well, and higher costs. There is a need for expanding the distance between acoustic sources and receivers, or utilizing additional receivers without increasing tool size.
The use of a magnetic clamping device as a method of attachment can also optionally be utilized to attach the sensors. However, the ability to magnetically clamp and unclamp the sensor downhole or at the well head does not resolve all retrieval problems because many times the tool, specifically the weight or main electronics cartridge, gets stuck downhole. Magnetic clamping alone will not address the issue of the stuck tool.
Accordingly, the present invention is operable to overcome one or more of the problems as set forth above.